Method of making a photographic product



Jan. 10, 1967 R. A. WILFERTH SILVER (MOLTEN) SILVER BROMIDE VAPORIZE 5-IO OF SILVER BROMIDE VAPOR DEPOSIT THIN FILM OF SILVER BROMIDE ON SUBSTRATE SUR FACE BROMINE (GAS) SILVER BROMIDE AND VOLATILE IMPURITIES INVENTOR ROBERT A.WILFERTII ATTORN F S United States Patent Ofiice 3,297,464 Patented Jan. 10, 1967 3,297,464 METHOD OF MAKING A PHGTOGRAPHIC PRODUCT Robert A. Wilferth, Pittsford, N.Y., assignor, by mesne assignments, to Technical Operations, Incorporated, a

corporation of Delaware Filed July 8, 1963, Ser. No. 293,288 8 Claims. (Cl. 11734) The present invention relates to silver halide photographic materials, and to the preparation of such materials and to the preparation of silver halide therefor.

Silver halide photographic materials in most prevalent usage today are formed as emulsion films, comprising grains of silver halide imbedded in a gelatin matrix or binder. In order to eliminate the gelatin or like binder material, to obtain materials having finer silver halide grains, and to obtain materials having a denser arrangement of the silver halide grains, certain efforts have been expended over the years to the development of photographic materials formed by the evaporation of silver halide and the condensation of the vapors on a substrate surface, to form thereon a stratum of light sensitive silver halide suitable for photographically recording images. These efforts are exemplified by the US. Patents to De Boer 1,970,496 and 1,999,088, and to Mansfeld 2,945,771, and the French Patent 1,267,623 to Lu Valle et al.

In general, the formation of an evaporated silver halide photographic material as taught by the foregoing cited patents comprises the positioning of a suitable photographic substrate material within a chamber, along With a charge of silver halide, evacuating the chamber to a high vacuum and heating and vaporizing the silver halide. The halide vapors condense on the surface of the substrate material and there collect as a stratum of photographic silver halide microcrystals. The microcrystalline grains that form on the substrate are bound there by adhesion directly to the substrate and by cohesion directly to each other. In this process, no gelatin or other matrix is normally provided as a binder for the grains, even though gelatin may be employed as the substrate material, or as a subbing layer on another substrate material.

The photographic material may of course be formed as individual plates, by exposing successive substrate plates to the evaporating silver halide vapor, or as a continuous strip by passing an indefinite length web over the evaporating silver halide charge at a rate adapted to collect a silver halide microcrystalline stratum of desired thickness over the surface of the web. In either event, in order to optimize the photographic product, it is desirable that the light response characteristics of various portions of an evaporation run be approximately uniform. The greater the uniformity, of course the better is the quality of photographic material.

It has been found that certain impurities in the silver halide charge, even when present in such trace amounts as only a few parts per million, evaporate from the silver halide with appreciable variations in concentrations and thereby afiect the light response characteristics of the deposited silver halide stratum in a nonuniform fashion, resulting in variations in the photoresponse characteristic of the material from one portion of an evaporation run to another. In particular it has been found that the presence of copper, iron, lead and nickel are particularly deleterious in this respect. When these elements are present in the silver halide charge in trace amounts even as low as or less than only about ten parts per million, all of these metals evaporate out of the charge at a much greater concentration at the beginning of an evaporation run than at the end. In some instances, in the early part of an evaporation run the concentration of these impurity metals is even greater in the evaporated stratum than in the original charge, while at the end of the run their concentrations are much reduced in both the evaporated stratum and the residual silver halide charge, to less than about one part per million, and in some instances they are even undetectable by spectrographic analysis.

It has been found that the photographic uniformity and reproductibility of evaporated silver halide materials can be greatly enhanced if the concentrations of the above recited impurities are reduced to a point where the concentration of these impurities in the eflluent vapors is substantially constant, or essentially nil. Quantitative analysis of these materials at these very low concentrations is not accurate, and reliable data is difficult to obtain. Therefore, the exact concentration of these impurities is not known. However, dramatically improved uniformity in photoresponse characteristics has been obtained by bringing the concentration of copper, iron, lead, and nickel in the silver bromide charge down to extremely low values, believed to approximate about one or two parts per million or less.

It is accordingly one object of the present invention to provide an improved photographic material, of the type formed by the evaporation of silver halide from a molten charge onto a suitable photographic substrate material; and particularly wherein the improvement resides in improved uniformity of photoresponse characteristics.

Another object of the present invention is to provide photographic material of this type, wherein the concentration of copper, iron, lead, and nickel impurities in the silver halide charge is at a level where their concentrations in the efiluent vapors are substantially constant.

Still another object of this invention is to provide photographic material of the foregoing type wherein the concentration of each of said impurities in said charge is approximately one or two parts per million, or less.

And a still further object of the present invention is to provide a photographic material of the foregoing type wherein the concentration of each of said impurities over the area of the material produced is nearly uniform.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description of the present invention. This detailed description is presented, of course, merely for purposes of illustration to enable a full and complete understanding of the invention, and is not to be interpreted in a limiting sense.

The drawing is a flow sheet of the process.

In practicing the art of manufacturing evaporated silver bromide photographic materials in the manner as particularly described in said French Patent 1,267,623 to Lu Valle et :al., in certain instances variations in photographic properties of materials formed under what appeared to be substantially the same physical conditions have been experienced. It has been discovered that these variations are due, at least in large measure, to the presence of varying concentrations of impurities in the evaporated silver bromide stratum deposited by the evaporation process. This problem occurs despite the fact that'very high purity silver bromide is employed as a starting material. This very high purity silver bromide was made by reacting high purity silver with high purity bromine gas, to produce silver bromide essentially in accordance with the process of Malinowski as described in The Journal of Photographic Science, vol. 8, 1960, pages 69-71. In outline, in this process, the bromine gas is brought into contact with a bed of particulate silver to form silver bromide. The reaction bed of silver is heated to a temperature suflicient to maintain the product salt molten and cause it to flow 3 through the bed and through capillary filters or the like to remove sludge materials. bromide is collected in a suitable container and cooled to solidity.

The general procedure followed in manufacturing evaporated silver bromide photographic material was to place a suitable charge of the foregoing silver bromide in a chemically clean tungsten crucible. Electrical connections were made to the tungsten crucible to heat the same, and it was located within a vacuum chamber. Also located within the vacuum chamber, an appropriate distance from the crucible, was a Web of suitable photographic substrate material carried by a pair of rolls, whereby the web may be passed over the crucible as it is advanced from one roll to the other. The Web material might, for example, be the usual photographic substrate materials, such as baryta pa-per, triacetate film, or Cronar film.

The vacuum chamber was then evacuated to a pressure selected preferably between about 10- and IO- mm. Hg, and the crucible was heated to melt the silver bromide charge and bring it to a temperature selected preferably between about 550 and 675 C. When the temperature and pressure reached desired values and were stabilized, the web was advanced from one roll to the other, thereby passing over the crucible and collecting the silver bromide vapors emanating therefrom. The rate of feed of the web was controlled to permit a desired thickness of the silver halide stratum to accumulate on the exposed surface of the web. For most photographic purposes, it is preferred that the silver halide stratum thickness be from about 0.1 to about 0.5 micron.

Utilizing high purity silver bromide fabricated in the manner above-described, and utilizing the evaporation procedure outlined above, despite every effort to maintain the physical parameters essentially constant, variations in photographic properties of the coated material obtained were observed. The following example illustrates these variations.

Example 1 Four different webs of photographic quality baryta paper, each 60 inches long and inches wide were separately coated with silver bromide in the manner abovedescribed, to a silver bromide thickness of about 0.2 micron. The evaporation temperature was about 605 C. and the evaporation pressure was between about and 10* mm. Hg. Three samples were taken from each web, one from the beginning of the run, one from the middle of the run, and one from the end of the run. The photographic speed and gamma for each sample was determined in accordance with conventional sensitometry techniques, and these values are set forth in Table A.

Using as the measure of variability, one half of the 95% confidence range as a percent of the average, i.e., 95 of an infinitely large sample would fall between 100% minus this percent and 100% plus this percent, where. 100% equals the average, it will be seen that in the foregoing runs uniformity for photographic speed is i102% and for gamma is i59%.

It has been found that despite the high purity of the silver bromide used, the variability in speed and gamma The emergent molten silver 7 demonstrated in the foregoing example is due, at least in large measure, to the trace impurities present in the silver bromide charge. From experiments, and through spectrographic analyses, it has been found that when such impurities as copper, iron, lead, nickel, and magnesium are present in the silver bromide charge in concentrations as low as of the order of ten parts per million, they nevertheless eveporate out of the crucible at widely different concentrations during the evaporation run. For example, a charge of silver bromide was placed in a chemically clean tungsten crucible and utilized in the foregoing procedures to effect twelve consecutive evaporations of the silver bromide onto separate glass plate substrates. Samples of the silver bromide were taken from the original charge, from the residual charge, and from each of the twelve evaporated films, and spectrographically analyzed. The detectable impurities present were found to be aluminum, silicon, copper, iron, magnesium, calcium, nickel, and cadmium or arsenic (cadmium andarsenic could not be distinguished from each other at the concentration involved). The first evaporated film showed the highest impurity content, and the original charge was second highest. All other films contained lesser amounts of impurities, and the residual charge after the twelve evaporations contained the least impurities. Only the first film showed nickel. Iron and copper were present on the first film, but only a faint trace showed in all others. Aluminum was detected only in the boat samples. In the case of iron, nickel, copper, and magnesium, neither boat sample showed as much as the first film. Other evaporations followed by comparative spectrographic analyses for the impurities in the evaporated film at the beginning of a run, at the end of a run, and in the residual silver bromide charge confirmed these results. In addition, those samples containing a small concentration of lead impurity showed that it acted like the copper, iron, nickel, and magnesium, in that it evaporated from the charge at a relatively higher concentration during the initial stages of the evaporation run, and then at a lesser concentration during the remainder of the run. Thus, it is discovered that the particular impurities copper, iron, lead, and nickel are particularly troublesome in this environment because of their relatively great volatility under the conditions of the present procedure, presenting a much exaggerated proportion of impurities in the evaporated layer during the early stages of an evaporation run. Indeed, with these four elements, it appears that almost all of each evaporates off during the early stages of evaporation of a charge.

It has been found that the uniformity of the photographic properties of the evaporated silver halide material can be greatly improved by appropriately depleting the silver halide of impurities. Two pretreatments of the silver halide before being used in the evaporation and deposition procedures described above are particularly effective. One is to leach the silver halide with distilled water by pelleting it in water from a molten state, and preferably in water that is hot and just under boiling temperature; and the other is to evaporate a small portion, i.e., from a few percent up to about 10%, of the charge before starting the coating operation.

The benefits of evaporating a small portion of the charge before commencing the coating operation are illustrated in the following example:

Example II A charge of silver bromide was prepared in the manner suggested hereinabove, this charge being comparable to that employed in Example I. Before preparing any photographic material therefrom, the charge was placed in the high vacuum evaporator and there exposed to a pressure of about l0- mm. Hg, and heated to a temperature of about 660 C. for ten minutes. This effected an evaporation of about 510% of the charge. With the charge thus pretreated, six different webs of gelatin subbed triacetate film, each 30 inches long and 5 inches wide were separately coated with silver bromide in the manner above-described, to a silver bromide thickness of about 0.1 micron. The evaporation temperature was approximately 581 C. and the evaporation pressure was between about 10- and lmm. Hg. Three samples were taken from each of the six webs, one from the beginning of the run, one from the middle of the run, and one from the end of the run. The photographic speed and gamma for each sample was determined, and these values are set forth in Table B.

Uniformity of the foregoing samples is found to be :55 for photographic speed, and i44% for gamma. It will be appreciated that these results represent a substantial improvement over the results shown in Example I.

The uniformity results can be further drastically improved by effecting a water leach of the silver halide before using it as a charge for preparing evaporated silver halide photographic materials. The advantages of this pretreatment are illustrated in the following example.

Example 111 Silver bromide was synthesized from high purity silver and bromine in the manner outlined above, except that the emergent molten silver bromide was collected by causing it to flow into a pool of triply distilled water held at just below the boiling point. As drops of the molten silver bromide hit the water, the drop pelletized with local boiling of the water, and crazing and fragmentation of the silver bromide pellets, producing a large surface interface between the bromide and the water for efficient leaching of impurities. The resultant silver bromide was then dried in air at 200 C. and used as a charge for coating gelatin subbed tn'acetate film in accordance with the procedure of Example H, including the precoating evaporation at about 660 C. for about ten minutes. One web of triacetate 30 inches long and 5 inches wide was coated in this manner with this silver bromide. Five samples were taken from this web at distributed points along its length and each sample was divided into two parts along the longitudinal center line of the web. The photographic speed and gamma for each part of each sample was determined, and these values are set forth in Table C.

TABLE 0 Sample Sample Part Speed Gamma 1 Above center line 017 1. 84 Below center line 018 2. 00 2 Above center line 018 1. 65 Below center line 019 1. 74 3 Above center line 016 1. 93 Below center line 020 1. 92 4 Above center line 020 1. 78 Below center line 019 1. 79 5 Above center line 019 1. 70 Below center line 019 1. 58

Uniformity of the foregoing samples is found to be :12% for photographic speed, and 122% for gamma.

6 These results represent a very substantial improvement over those obtained in Example II, and a most demontsrative improvement over those obtained in Example I.

By Way of further comparison, to demonstrate the advantages of water pelleting of the silver bromide apart from a precoating evaporation cut of the silver bromide charge, a number of runs have been made wherein the evaporation coating was conducted from the initial melting of the silver bromide charge, and the speeds of the resultant photographic materials were determined as a function of percentage of silver bromide charge evaporated. In some of these runs, the silver bromide charge was formed by pelleting the molten silver bromide material in distilled water, as described in Example 111. In others, the molten synthesized silver bromide was merely pelletized and collected in air. These evaporations were run with a charge temperature of about 660 C., at a pressure of about 10- mm. of Hg, and deposited onto a moving web of baryta paper to a thickness of about 0.15 micron. With either method of preparing the silver bromide, it was found that the speed of the photographic material rose very rapidly during the early stages of the evaporation and then leveled off to a relatively uniform value. Those charges that were prepared by pelleting in distilled water leveled off to a relatively uniform speed rather early in the evaporation run, usually after about 35% of the original charge had been evaporated; while the other charges, those that were merely collected in air, leveled oif to a relatively uniform speed only after evaporation of about 510% of the original charge had been evaporated. By spectrographic and polarographic analyses, it was found that the water pelleting procedure per so greatly reduced the iron and cadmium content of the silver bromide.

As stated previously herein, the improvement in uniformity of photographic properties .above demonstrated is due at least in large measure to the reduction of volatile impurities to a very low level in the silver bromide charge. By the foregoing pretreatments of the silver bromide charge the concentration of impurities, particularly copper, iron, nickel, and lead, is reduced to a point where the concentration of these impurities in the deposition on the substrate during the evaporation and coating procedure is substantially constant, or essentially nil. The exact concentration of these impurity elements cannot be stated with certainty, because of the difiiculty in obtaining reliable quantitative data at such low concentrations. However, it is believed that the concentration of these elements in the silver bromide charge, after the two described pretreatments, is about one or two parts per million, or less.

The results attributable to the precoating evaporation cut cannot be explained as a simple matter of volatilization of the impurities, because it has been observed that the same improved uniformity results are not obtained if the cut is efi'ected at or around the same temperature as the coating evaporation, even if the precoating evaporation cut is taken for a much longer time and of a much greater percentage of the silver bromide charge. The best improvements are obtained by conducting the precoating evaporation cut at a temperature substantially in excess of that at which the coating evaporation is to be conducted, preferably between about 50 and about C. in excess thereof.

The present invention has been herein characterized and defined in terms of impurity level and uniformity of impurity level. In addition it has been observed from electron photomicrograph studies of silver bromide layers formed in the manner set forth in Examples I and II, that the crystals or grains formed by the latter procedure are several times larger than those formed by the first procedure, and the latter produces grains or crystals that appear to be somewhat more uniform in size than the first procedure.

There have thus been presented specific examples of the procedures of the present invention for obtaining improved uniformity of evaporated silver halide photographic materials. It is understood, of course, that the objective of these procedures is to obtain a uniform level of impurity in the evaporation deposited layer of silver halide and resultant uniform photographic property. It is therefore contemplated, and not intended to be excluded from the scope of this invention, that the impurity level of the silver bromide charge may be reduced, as herein described for example, and then impurities may be added in uniform concentration to the silver bromide vapors in a manner designed to obtain uniform sensitization of the deposited silver bromide. Also, it is not intended to exclude from the scope of this invention the application of uniform concentrations of sensitizer impurities to the surface of the substrate web or to the surface of the deposited silver halide stratum. Accordingly, such variations and modifications of the present invention as are embraced by the spirit and scope of the appended claims, are contemplated as within the purview of the present invention.

What is claimed is:

1. In the method of producing silver halide photographic material wherein silver halide, containing impurities, is evaporated from a molten pool under high vacuum conditions, and the efiiuent vapors are condensed on a substrate layer to form on said layer a stratum of silver halide microcrystals, said microcrystals being adhered directly to said substrate and cohered directly to each other; the improvement wherein prior to producing said photographic material a minor percentage of said silver halide is pre-evaporated under a vacuum approximating, and at a temperature substantially in excess of, the conditions of evaporation during production of said photographic material.

2. In the method of claim 1, said minor percentage being from about to about 10% by weight.

3. In the method as set forth in claim 1, said silver halide being silver bromide.

4. In the method as set forth in claim 3, the temperature of said silver bromide during the coating evapora- 8 tion being between about 550 C. and 675 C., and the pressure being between about 10' and 10" mm. Hg.

5. In the method as set forth in claim 1, said impurities being copper, lead, iron, and nickel.

6. A process of preparing a silver halide photographic material, comprising reacting silver with a halogen gas to produce silver halide, pre-evaporating a minor percentage of said silver halide at a temperature at which the silver halide is molten and under high vacuum conditions, thereafter evaporating said pre-evaporated silver halide under substantially similar conditions of pressure and at a lesser temperature and condensing the silver halide vapor on a surface of a substrate material to form a photographically responsive stratum of silver halide microcrystals on said substrate material.

7. A process as set forth in claim 6, wherein said silver halide is silver bromide, s-aid halogen gas is bromine, said minor percentage is between about 5% and 10% by weight, the temperature and pressure of evaporation during coating are between about 550 C. and 675 C. and between about 10* and 10- mm. Hg, and the thickness of the stratum is between about 0.1 and 0.5 micron.

8. In the process as set forth in claim 4, said minor percentage being from about 5% to about 10% 'by weight, and the thickness of the stratum is between about 0.1 and 0.5 micron.

References Cited by the Examiner UNITED STATES PATENTS 2,945,771 7/1960 Mansfeld 11734 3,219,448 11/1965 Lu Valle et al. 1l734 3,219,450 11/1965 Goldberg 9694 BF FOREIGN PATENTS 1,267,623 12/1961 France.

OTHER REFERENCES Malinowski: J. Photo, Sci. 8, 69-71 (1960). West: J. Phy. Chem., 63, -54 (1959).

MURRAY KATZ, Primary Examiner. 

1. IN THE METHOD OF PRODUCING SILVER HALIDE PHOTOGRAPHIC MATERIAL WHEREIN SILVER HALIDE, CONTAINING IMPURITIES, IS EVAPORATED FROM A MOLTEN POOL UNDER HIGH VACUUM CONDITIONS, AND THE EFFLUENT VAPORS ARE CONDENSED ON A SUBSTRATE LAYER TO FORM ON SAID LAYER A STRATUM OF SILVER HALIDE MICROCRYSTALS, SAID MICROCRYSTALS BEING ADHERED DIRECTLY TO SAID SUBSTRATE AND COHERED DIRECTLY TO EACH OTHER; THE IMPROVEMENT WHEREIN PRIOR TO PRODUCING SAID PHOTOGRAPHIC MATERIAL A MINOR PERCENTAGE OF SAID SILVER HALIDE IS PRE-EVAPORATED UNDER A VACCUM APPROXIMATING, AND AT A TEMPERATURE SUBSTANTIALLY IN EXCESS OF, THE CONDITIONS OF EVAPORATION DURING PRODUCTION OF SAID PHOTOGRAPHIC MATERIAL. 